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SULPHUR                                       115

with hydrogen sulphide and filtered and the precipitated copper sulphide,
with any lead, bismuth and antimony sulphides, rapidly washed. The
precipitate is then dropped into a conical flask and redissolved in nitric
acid, with which the filter is well washed. The solution is evaporated to
dryness and the residue taken up in water and a little nitric acid, neutralised
with ammonia, and dilute sulphuric acid added to precipitate the lead. The
liquid is then filtered and the insoluble residue well washed, the filtrate being
treated with 3-8 c.c. of nitric acid (D 1-40) and subjected to electrolysis to
separate the copper. The weight of copper found is diminished by o-oi
gram to allow for any bismuth and antimony present.

Another method, more convenient and rapid, is as follows :
Five grams of the very finely ground pyrites are carefully calcined in a
porcelain dish at a dull red heat and are mixed until completely roasted
(the arsenic is expelled and the tin rendered insoluble), the product being
boiled for 15 minutes Math 30 c.c. of nitric acid (D 1-42) in a 250 c.c. flask.
When cool, the liquid is made up to the mark with water, shaken and fil-
tered, 200 c.c. of the filtrate (— 4 grams of substance) being neutralised with
ammonia, mixed with* 5 c.c. of cone, nitric acid and subjected to electro-
lysis (see Copper Sulphate, 3, a}.

With chalcopyrite and products containing more than 15% Cu, 2 grams
are taken for analysis.

4.  Arsenic (Reich and McCay's method).—0-5 gram is heated almost
to dryness with cone, nitric acid in a porcelain basin, 4 grams of sodium
carbonate being then added and the evaporation continued to dryness.
The residue is then fused and kept fused for 10 minutes with 4 grams of
nitre.    When cold, the mass is taken up in hot water and the liquid filtered,
acidified with nitric acid, boiled to expel all the carbon dioxide, treated
with silver nitrate and neutralised with dilute ammonia.    The precipitate,
which contains all the arsenic as silver arsenate, is filtered off, washed well
with water and redissolved in dilute nitric acid, the solution being evaporated
to dryness in a tared platinum basin.    The residual silver arsenate is either
weighed or titrated with thiocyanate according to Volhard's method for
determining silver :   i part of Ag3As04 = 0-162 part of As ;  i part of Ag
= 0-2315 part of As.

5.  Lead.—The insoluble residue remaining after the treatment with
aqua regia for the determination of sulphur (see 2) is treated with a hot,
concentrated ammonium acetate solution, the liquid evaporated to dryness
with a little sulphuric acid, and the lead sulphate—thus separated from other
insoluble matter—ignited and weighed : i part PbS04 = 0-6832 part Pb.

* *

Sicilian Mineral sulphur may contain up to about 90 % of sulphur, but high
proportions (above 40%) are rare. Those containing 30-40% are usually re-
garded as rich, 20-30% as good, 15-20% as ordinary, and less than 15% as poor.
Most of it is yellow tending to grey or greenish, but some is brown owing to
bitumen. Those of Romagna contain, on the average, 10-20%, and occasion-
ally 30% of sulphur, and are often bituminous. Those of Louisiana are very
rich (60-98%), while those of Nevada, Utah, Texas, etc., contain varying pro-
portions (15-80%).

Crude sulphur usually contains 98-99-5% of sulphur.    The moisture rarely                             7n aly for delicate colours, should not contain iron.                                                                       l.2o